1. Field of the Invention
2. Description of Prior Art
Biological treatment is a useful method for the destruction of organic compounds in groundwater and wastewater. Bacteria or other microorganisms which are effective for biological treatment may be used in free suspensions, or alternatively in biofilms fixed on solid supports. Use of solid supports typically reduces both cell washout (in flow systems) and sludge formation, and may enhance biological treatment in other ways. Supports which have non-biological (e.g., adsorptive or catalytic) functions may afford expanded capabilities for wastewater or groundwater treatment.
Activated carbon is a common support for bacteria and other microorganisms to be used in the biological treatment of contaminated water. (By "activated carbon" is meant any porous, high surface area form of carbon.) For example, [R. F. Hickey, D. Wagner, and G. Mazewsi, "Combined Biological Fluid Bed-Carbon Adsorption System for BTEX Contaminated Groundwater Remediation," presented at the Fourth National Outdoor Action Conference on Aquifer Restoration, Groundwater Monitoring and Geophysical Methods, Las Vegas, Nev., May, 1990] teaches the use of biofilms on granular activated carbon for the degradation of benzene and alkylbenzenes in groundwater. The ability of activated carbon to adsorb organics from water enhances its utility as a support. Thus, when organics are too dilute to serve as a carbon source for bacteria or other microorganisms, they may be concentrated on the carbon to a level where they can be degraded. Further, carbon can protect the biofilm in a bed from transient toxic levels of organics (shocks) by adsorbing these, and later slowly releasing the organics at much lower levels. Even if substantially all of the bacteria or other microorganisms in a bed are killed by a shock, the carbon support will adsorb pollutants while the carbon is being recolonized, and so prevent even temporary breakthrough of the pollutants.
Activated carbon has a number of deficiencies which militate against its use. First, carbon typically is not mechanically strong, and so is broken up easily during handling or fluidization. Second, carbon is very light, and is difficult to keep in fluidized beds. A third disadvantage of activated carbon is that it is microporous, and typically has very few, if any, macropores. Biofilms thus may be formed only on the external surfaces of carbon particles, while potentially higher biomass loadings are possible on materials which have pores large enough to contain bacteria and other microorganisms. Finally, activated carbon by its nature has particular surface chemical properties. For some applications, other surface chemistries may be desired, e.g., some bacteria may prefer to colonize more hydrophobic surfaces than that of carbon.
Many non-carbonaceous materials also have been used to support biofilms. In this category are solid inorganic oxides, which may have a number of advantages over activated carbon. These include good mechanical strength and attrition resistance, densities suitable for use in fluidized beds, the ability to tailor surface chemical and physical properties by the choice of oxide, the ability to add other properties, such as buffering or ion exchange, and the possibility of having pores with diameters large enough to permit microbial colonization. This last property offers the possibility of establishing cell densities (cells/unit volume) much higher than those available in materials such as carbon in which only the external geometric surface area is available for biocolonization. Our co-pending applications U.S. Ser. No. 994,222, filed Dec. 21, 1992, and U.S. Ser. No. 997,261 filed Dec. 21, 1992, the contents of which are incorporated here by reference, describe the preparation and use for bioremediation of clay-based supports with micron-sized pores.
Despite these advantages, most inorganic materials will not have the spectacular ability of activated carbon to adsorb organic compounds. As was noted above, this ability greatly enhances the efficacy of biofilms supported on carbon for biological treatment of aqueous streams.
Thus, existing supports, composed either of carbon or of inorganic oxide materials, have one or more deficiencies for use in biotreatment. Supports which possess the best properties of both are desired. These properties include the adsorptive capacity of carbon and the strength, density, potential large pore diameters, and tailorable chemistry, e.g. buffering capacity, of the other inorganic materials. These supports further should be easily prepared.